I don't doubt that Spotify is using internal memory.
What confuses me is the first part of you post.
How do you know that data is being saved to SD? (what evidence do you see, what does Android "show", using which function or app?)
Is the data being duplicated in both internal and SD?
A bit more info would help.
And which version of Android is on your devices?
Hey @Bigmacofcville it sounds like the bug I reported here.